Mechanical and tribological properties of diamond-like carbon coatings prepared by pulsed laser deposition

8URTACE

COATINGS

HE#/IOLOGY ELSEVIER

Surface and Coatings Technology 76-77 (1995) 534-539

Mechanical and tribological properties of diamond-like carbon coatings prepared by pulsed laser deposition A.A. Voevodin ~, M.S. Donley ~, J.S. Zabinski ~, J.E. Bultman b WL/MLBT, Materials Directorate, Wright Patterson AFB, OH 45433-7750, USA u Research Nstitute, University of Dayton, Dayton, OH 45469, USA

Abstract The tfibological properties of diamond-like carbon (DLC) coatings produced by pulsed Iaser deposition (PLD) are investigated. Films are grown onto steel substrates to 0.5 gm using a 248 nm laser to ablate graphite and polycarbonate targets in high vacuum. Chemical bonding is studied with Raman, XPS and EELS techniques; mechanical and tribological properties are evaluated using microindentation and ball-on-disk friction tests. Coatings grown from graphite targets are amorphous DLC (a-C), while those grown from polycarbonate targets are amorphous hydrogenated carbon (a-C:H). The hardness of the a-C coatings is 55-70 GPa and the hardness of the a-C:H coatings is 12-20 GPa depending on the substrate bias. Friction coefficients of the coatings against steel and sapphire balls are determined in several environments: in air as a function of relative humidity, in dry nitrogen, and in 10 Pa vacuum. For a-C coatings, the friction coefficients are typically below 0.1 and are as low as 0.03 in dry nitrogen. In wear tests, a critical contact pressure of 1.4 GPa led to catastrophic adhesive failure of a-C coatings, whereas failure of a-C:H coatings is by wear-through after 5 x 103 cycles. Extremely low wear rates of 10 .9 mm 3 N -~ m -~ are found for a-C coatings at the contact pressure of 0.8 GPa. Keywords: Mechanical properties; TriboIogicaI properties; Diamond-iike carbon; Coatings; Pulsed laser deposition

1. Introduction

2. Experimental details

High hardness, low friction, and low wear are recognized benefits of diamond-like carbon (DLC) for its application as tribological coatings Eli. A number of different techniques have been successfully applied to deposit wear- and friction-reducing DLC coatings E2,3]: among them pulsed laser deposition (PLD) is an emerging technology. PLD can be used to produce carbon coatings with structures ranging from hydrogenated amorphous carbon (a-C:H) [ 4 - 7 ] to amorphous carbon (a-C) [8-12] and even "nanophase diamond" [13-15] with hardness as high as 80 GPa. Another advantage of P L D is precision in coating composition and low deposition temperatures (25-100 °C) E16]. A recent literature review by the authors [ 17] shows almost an absence of published work on friction and wear properties of DLC coatings grown by PLD. We started such characterizations with both a-C and a-C:H coatings: the first results are reported here. This work provides a comparison of hardness, elastic modulus, friction coefficients, load support capability and wear rates of a-C and a-C:H coatings prepared by the pulsed laser ablation of graphite and polycarbonate targets, respectively, in high vacuum conditions.

2.1. Coating preparation

0257-8972/95/$09.50© I995 ElsevierScienceS.A. All rights reserved

The equipment configuration used to deposit coatings includes a stainless steel U H V chamber with target and substrate holders, an excimer laser, and beam scanning and focussing systems. A Lambda Physic LPX 110i laser charged with KrF is operated at 20 Hz and 200 mJ pulse -1 to provide laser pulses of 20 ns width, which are focused on the target surface to give ~ 109 W cm -2 photon energy density. Targets of superconductive grade graphite or polycarbonate ECIaO3H14],, are used to deposit a-C and a-C:H coatings, respectively. Disks of 25.4 mm diameter 440C steel polished to 1.0 ~tm are used as substrates. They are ultrasonically cleaned in acetone, then placed in the chamber and etched in an argon glow discharge at 1 kV negative bias for 30 min. The chamber is evacuated to a base pressure of 10 .6 Pa before the coating deposition starts. The irradiation of targets results in an increase of pressure during deposition to 10 -s Pa and 10 . 4 Pa for graphite and polycarbonate targets, respectively. The a-C coatings are grown at 0 and - 1 0 0 V bias while the a-C:H are grown at 0 and

A.A. Voevodin et aL/Smface and Coatings Technology 76-77 (1995) 534-539

- 8 0 0 V bias. Temperature during deposition is in the range 60-90 °C. The substrate to target distance is 6 cm. Under these conditions, coatings of 0.5 gm thickness are deposited at typical growth rates of ~0.01 gm min -~ for a-C coatings and ~0.09 tam rain -1 for a-C:H coatings.

2.2. Characterization procedure Immediately after deposition, X-ray photoelectron spectroscopy (XPS) is performed on the coatings. Some coatings are also investigated with Raman and electron energy loss spectroscopy (EELS). The hardnesses and elastic moduli of the coatings are determined with a Nanoindenter ® microprobe. A Berkovich indentor loaded in the range from 0.7 to 8.9 mN is used to perform microindentations. Procedures developed by Oliver and Pharr [-18] are applied for both instrument calibration and data analyses, which allows the minimization of indentation size effect (ISE) at small penetration depths. Friction and wear tests are performed using a ballon-disk tribometer. For these tests, 3.2mm diameter balls manufactured from 440C steel with 8 GPa hardness and sapphire with 18 GPa hardness are used. Tests are conducted in air at controlled relative humidity (RH), in dry nitrogen atmosphere, and in 10 Pa vacuum. All tests are run at room temperature. Initial mean contact pressures in the range 0.6 to 1.4 GPa are used in wear tests. The pressures are calculated based on actual loads using the assumption of the Hertzian contact and ignoring the influence of the coating. A sliding speed of 0.2 m s -I is used in all experiments. One turn of the disk corresponds to approximately 75 mm of sliding. Friction coefficients (/1) are recorded for the different tests; an increase of# above 0.3 is considered as a coating failure. An optical microscope and a Dektak profilometer are then used to measure the diameter of ball wear spots and the diameter, depth and width of wear tracks on coated disks. Based on these measurements, linear wear rates (#m) are determined from the changes in ball diameter and wear track depth, and normalized wear rates (mm 3 m-1 N - l ) are calculated from the wear volumes divided by sliding distances and applied absolute loads.

For a-C:H coatings only one 284.4 eV peak is recorded, suggesting the coatings are more graphitic. EELS data obtained for valence electron levels show an absence of energy losses in the 7 eV range (associated with graphite) for a-C:H coatings and very small losses in this range for a-C coatings. At the same time both coatings have broad maxima at 25-35 eV, suggesting the presence of diamond type bonds in the coatings. Based on the above data, the a-C coatings are characterized as DLC while the a-C:H coatings deposited from polycarbonate targets require more detailed investigations before conclusions about their structure can be made.

3.I. Hardness and elastic modulus Results of hardness measurements are given in Fig. 1 as a function of load applied to the indentor. The values of hardness determined for fused silica at the same conditions are used as an internal reference and are plotted in Fig. 1 as well. The hardness of fused silica is nearly constant for loads higher then 1 raN, indicating very small ISE. The hardness of a-C coatings and, to a lesser extent, a-C:H coatings varies with load, and it is difficult to determine the actual hardness of the coating. At minimized ISE the main influence on hardness variation is due to the substrate, so a penetration depth of 1/10 of the coating thickness can be used for hardness evaluation. The penatration depth of 1/10 of the thickness is reached at loads of approximately 2 mN for a-C coatings and 1 mN for a-C:H coatings. For these loads, Fig. 1 gives hardness values of 55-70 GPa for a-C coatings and 12-20 GPa for a-C:H coatings, depending on biasing condition. Corresponding elastic moduli of 550-560 100 -

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A.A. Voevodin et aL/Surface and Coatings Technology 76-77 (1995) 534-539

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GPa for a-C coatings and 120-220 GPa for a-C:H coatings are found at these indentation ioads.

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3.2. Friction coefficients

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Friction coefficients of coatings are determined in sliding against both 440C and sapphire balls at the load of 100 g, which corresponded to Hertzian contact pressures of 0.6 GPa for steel balls and 0.8 GPa for sapphire balls. To achieve stabilized friction, typically 103 turns are done before it is recorded. All experiments are repeated to verify reproducibility. The difference between results from duplicate samples is typically within 10% of the absolute friction values, except for a-C coatings prepared in the biased conditions, for which the adhesion instability caused a variation of 20%-30%. Table 1 summarizes results for friction coefficients of a-C and a-C:H coatings obtained in different environments. For all conditions/~ is in the range 0.03-0.12 for a-C coatings and 0.05-0.26 for a-C:H coatings. The following four important observations are made: (i) friction coefficients of a-C:H coatings are higher than those of a-C coatings; (ii) for a-C coatings, the difference between friction with steel and sapphire balls is much less than that for a-C:H coatings, indicating that the tribobehaviour of a-C coatings is less sensitive to counterpart material; (iii) all coatings have higher friction coefficients in a vacuum of 10 Pa than in dry nitrogen and humid air; (iv) for both a-C and a-C:H coatings the application of negative bias during film growth results in reduced friction. The influence of relative humidity on the coating friction is then investigated. The results for sliding against steel balls are presented in Fig. 2 and for sliding against sapphire balls in Fig. 3. In both cases, a reduction of/~ with increasing RH is observed. The effect of humidity is especially important for a-C:H coatings in sliding against steel balls (Fig. 2).

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3.3. Load support capability Table 1 Comparison of friction coefficients of a-C and a-C:H coatings prepared by pulsed laser deposition under different biasing conditions Friction test environment

a-C:H non-biased

a-C:H biased

a-C non-biased

a-C biased

In sliding against 440C Air, 50% RH Nitrogen, <2% RH Vacuum, 10 Pa

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For the present study, load support capability (LSC) is defined as the capability of a coating-substrate pair to withstand local contact pressures at conditions of sliding friction. Thus, it is as much a function of coating mechanical properties as a function of coating adhesion and compliance of the substrate. Since the same substrates and coating thickness are used in these tests, it is possible to make relative comparisons of LSC for the coatings. Any change in LSC could be related to mechanical and adhesion properties of the coatings. The coating LSC is studied by direct observations of wear tracks and measuring linear wear of coated disks after 5 x 103 sliding turns against steel balls with contact

A.A. Voevodinet al.ISmfaee and Coatings Technology 76-77 (1995) 534-539

Fig. 4 presents data only for non-biased variants of the coating preparation. Biased a-C:H coatings showed behavior similar to that of non-biased, and biased a-C coatings failed after several hundred turns at contact pressures higher than 0.6 GPa by an adhesion failure mechanism.

pressures of 0.4, 0.6, 0.8, 1.1 and 1.4 GPa. Four tests for each pressure are performed. Results are illustrated in Fig. 4 with optical micrographs of typically observed wear tracks for some of the contact pressures. A contact pressure of 1.4 GPa is found to be critical for both a-C and a-C:H coatings but failure is by different mechanisms. Linear wear of a-C:H coatings increases from 0.02 t o 0.05 Ixm in the 0.4-0.8 G P a pressure range. Further increase of the contact pressure to 1.1 G P a results in linear wear of 0.15 Ixm and coating wearthrough at 1.4 GPa. Wear of a-C coatings is below the measurement error of 0.01 lxm for pressures up to 1.4 GPa, when sudden failure occurs by coating fracture and delamination.

3.4. Wear rates

To evaluate wear rates, extended experiments are performed with sapphire balls at a pressure of 0.8 GPa in an air environment with 30% RH for 2 x 10s turns. Four experiments are run for each condition to provide average results. Table 2 summarizes the results of these

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A A. Voevodin et al./Swface and Coatings Technology 76-77 (1995) 534-539

Table 2 Comparison of ball and coating wear in sliding steeI disks coated with a-C and a-C:H coatings against sapphire balls Test parameter

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a-C:H biased

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N u m b e r of revolutions before film failure Bali linear wear rate (gin/revolution) Film linear wear rate (gin/revolution) Ball normalized wear rate (mm 3 m-X N - I ) Film normalized wear rate (mm 3 m -1 N - l )

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Tests were performed in an air environment with 30% RH, at contact loads of 100 g (0.8 GPa) and sliding speed of 0.2 m s -1.

experiments. Only orders of magnitudes are given for calculated normalized wear rates, owing to the uncertainty in wear volume calculations. Steel balls are not used for this type of experiment, because their extensive wear against a-C coatings made comparison of results impossible. Biased a-C coatings are also not considered due to their unstable behavior associated with poor adhesion. Both biased and non-biased a-C:H coatings had the same wear rates, and coating thicknesses are worn through after approximately 1.5 x 104 turns. Normalized wear rates of a-C coatings are 2-3 orders of magnitude lower than that of a-C:H, but are accompanied by remarkably higher wear of the sapphire ball. The observation of wear tracks on the surface of the a-C coating suggests that they correspond to "steady state" sliding friction even after 2 x 105 turns (~15 km of sliding distance). Their typical appearance is similar to that given in Fig. 4 for the a-C coating at 0.8 GPa contact pressure: Examination of sapphire balls after 2 x 10s turns against a-C coatings reveals well distinguished wear spots with the formation of a transfer film.

have mechanical properties close to those of DLC produced by ion beam [23,24] and filtered arc techniques [25]. The high hardness of these coatings is accompanied by low friction coefficients for a wide range of conditions. An increase in friction with increasing humidity for coatings with a high fraction of diamond bonds in their structure is observed in a number of works [26-28]. In this research the opposite dependence is found for both a-C and a-C:H coatings. At present, we do not have enough information on the surface chemistry in wear tracks to unambiguously determine the mechanism for the behavior in humid air. However, a possible speculation is the transition of the friction surface of the a-C coatings into graphitic-like material after 103 turns. Similarly, a higher graphite content in a-C:H coatings may account for the observed dramatic influence of RH on their friction coefficients against steel. The wear rates of a-C:H coatings of around 10 .6 mm 3 N -1 m -I found here are on the high side of the range reported for DLC coatings [26,29,30]. In contrast, the wear rates of a-C PLD coatings are extremely low, which is probably a result of their high hardness in combination with low friction coefficients. When compared with wear rates of DLC coatings produced by other techniques, e.g. 10-6-10 .7 mm a N -~ m -I for DLC produced by plasma assisted chemical vapor deposition [26,29] or 10 .6 mm a N -I m - i for sputtered metal-DLC composites [30], the wear rates of the a-C PLD coatings of 10 -9 mm a N -1 m -~ make them attractive for wear-reducing applications. However, the usage of a-C PLD coatings at high contact pressures is limited by insufficient load support capability, originating partly from poor adhesion strength to steels and partly from inadequate mechanical properties of the 440C substrates. Thus a supporting intermediate layer is needed for DLC coatings deposited on steel substrates. The discsussion of a potential supporting intermediate layer with gradually changing properties across the thickness is provided in Refs. [30,31] and different aspects of the multilayer coating development are analyzed in Refs. [32,33].

4. Discussion

The observed difference in the mechanical properties and tribological behavior of a-C and a-C:H coatings cannot be associated only with hydrogen presence in the coatings, but more likely is a result of different plasma compositions when using a graphite or a polycarbonate target [22]. Incorporation of large molecular clusters into the a-C:H coatings could be another reason for their lower hardness. High hardnesses and elastic moduli of a-C coatings produced by PLD has been reported by other researchers [8-12]. When compared with other techniques of low temperature DLC deposition, the a-C PLD coatings

5. Conclusions

Mechanical and tribological properties of a-C and a-C:H coatings produced by PLD from graphite and polycarbonate targets are considerably different. While the hardnesses and elastic moduli of the a-C coatings are 55-70 GPa and 500-650 GPa, respectively, the hardnesses of a-C:H coatings are in the range 12-20 GPa and moduli are in the range 120-220 GPa. Friction coefficients of a-C coatings (0.03-0.12) and a-C:H coatings (0.05-0.26) depend on the counterpart material and the test environment. Both coatings show a slight

A.A. Voevodin et aL/Swface and Coathzgs Technology 76-77 (1995) 534-539

decrease in friction with increasing humidity. However, for the a-C:H coating in sliding against steel balls, a dramatic decrease from 0.26 to 0.16 is observed. The application of negative substrate bias during coating growth resulted in minor increases in hardness and decreases in friction for both coatings. A critical contact pressure of 1.4 GPa is found for the coatings, which resulted in catastrophic adhesion failure of a-C coatings and wear-through failure of a-C:H coatings after 5 x 103 cycles. For a-C coatings the extended wear tests at 0.8 GPa contact pressures gave extremely low normalized wear rates of 10-gram 3 N -1 m -1. The tribological properties of PLD DLC coatings compare favorably or exceed those of DLC coatings produced by other techniques, indicating that they have good potential in applictaions requiring low friction and low wear of surfaces.

Acknowledgments The authors are pleased to thank the staff and facilities of Wright Laboratory Materials Directorate. We especially wish to thank Dr. J. Solomon for performing EELS investigations and Dr. S.J.P. Laube for PLD control developments. This work was performed while one of the authors (A.V.) held a National Research Council USAF/WL Research Associateship.

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